ACC97: Why a Calculated Control Is Blank

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When you have a calculation that contains a Null value, the result of the calculation will also be Null. A Null value is not the same as zero; instead, it can be thought of as an unknown value. The result of adding an unknown value to a number is unknown.

When you have fields that contain Null values, you can create an expression that converts the Nulls to zero. You might do this if you want the records containing Null values to be included in an aggregate (totals) calculation, or if you want to prevent an expression from resulting in a Null value when a field that is referenced in the expression contains Nulls. To convert Null values to zero, use the Nz() function. For example: